Commercial polyethylenes generally fall into one of two general categories based on their processability and their product properties.
Processability is the ability to predict and economically process and shape a polymer uniformly. Processability involves such elements as thermal stability, how easily the polymer flows, melt strength, and whether or not the extrudate is distortion free. Linear polyethylene (LPE) is more difficult to process than low density polyethylenes (LDPE) because LPE's are not as thermally stable, LPE's require more motor power and produce higher extruder pressures to match the extrusion rate of LDPE's. LPE's also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, however, LPE's exhibit superior physical properties as compared to LDPE's.
In order to take advantage of the superior physical and mechanical properties of LPE's, expensive antioxidants and processing aids must be added to the polymer, and extrusion equipment must be modified to achieve commercial extrusion rates.
It is common practice in the industry to add low levels of an LDPE to an LPE to increase melt strength, to increase shear sensitivity, i.e., to increase flow at a given horse power; and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat LPE.
A second technique to improve the processability of LPE's is to broaden the products' molecular weight distribution (MWD) by blending two or more LPE's with significantly different molecular weights, or by changing to a polymerization catalyst which produces broad MWD. The physical and mechanical properties and the processability of these broader MWD products are generally intermediate between those of narrower LPE components and LDPE/LPE blends with equivalent Mw.
Therefore, a need exists for catalyst which offers the processability of blends but the properties of the linear blend components.